This research is designed to obtain detailed information on the mechanisms by which lactogen (prolactin and placental lactogen) are involved in regulation of lactogen receptor in uterus during pregnancy. This information will be useful in evaluating the role of lactogens and lactogen receptor in maintenance of pregnancy and fetal development. The relationship of progesterone to lactogen receptor will also be evaluated. The approach to this problem involves incubation of pregnant and pseudopregnant rabbit uterine explants with placental explants and various concentrations of lactogen and/or progesterone which will allow evaluation receptor under controlled conditions. A second major focus of this research is the definitive purification of lactogen receptor from rabbit endometrium by chromatographic methods. After purification this receptor will be characterized kinetically and physiochemically. Antibody to this receptor will be prepared from guinea pigs characterized by immunodiffusion and be tested for its ability to inhibit the binding of oPRL and GH to their respective receptors in several rabbit tissues. The third major area of fucus of this project will be an examination of lactogen receptor in endometrial and corpus luteal membranes of mid-pregnant rabbits using both labeled prolactin and labeled prolactin antibody as probes. This section of the project will be approached by crosslinking hormone and/or receptor antibody to the membrane-bound receptor with disuccinimidyl suberate and evaluation of receptor size before and after reduction with dithiotheitol by means of sodium dedecyl sulfate-polyacrylamide gel electrophoresis. Non-denatured receptor prepared in a similar manner using dithiobis (disuccinimidyl propionate or ethyl glycol bis (succinimidyl succinate will be evaluated kinetically. These methods will allow evaluation of the characteristics of lactogen receptor under semibiological conditions. These approaches will allow both evaluation of receptor properties and an examination of regulatory factors in uterus and ovary.